Icarus 179 (2005) 523–526 www.elsevier.com/locate/icarus

Accurate absolute magnitudes for objects and Centaurs

W. Romanishin a,∗,1, S.C. Tegler b,1

a Department of Physics and , University of Oklahoma, Norman, OK 73019, USA b Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ 86011, USA Received 31 March 2005; revised 18 June 2005 Available online 15 August 2005


Accurate absolute optical values (HV and HR) for Kuiper belt objects (KBOs) and Centaurs are becoming increasingly impor- tant as observations in other wavelengths, particularly SIRTF thermal measurements, become available for large samples of objects. We present accurate HV and HR values for 90 KBOs and Centaurs, based on our published optical . We find that our HV values are in good agreement with those available from the European photometric survey of minor bodies in the outer . Comparison with HV values from the JPL Horizons database and the Minor Center database shows that these sources are systematically brighter than ours by about 0.3 mag.  2005 Elsevier Inc. All rights reserved.

Keywords: Centaurs; Kuiper belt objects; Trans-neptunian objects

1. Introduction magnitude is one of the most basic observable quantities of a minor Solar System body. There are now about 1000 cataloged minor outer So- Several groups are using infrared measurements of KBOs lar System bodies, with about 500 having been observed at and Centaurs, these being obtained with SIRTF, to measure more than one opposition. For the vast majority, nothing is the thermal emission properties of a subset of the population. known of their intrinsic physical properties. In 1995, we be- These observations should yield important new information gan a large and systematic program to obtain optical (BVR) on the of these objects. Comparison of the infrared and colors and magnitudes for a sample of these objects. Our optical fluxes can yield the optical . program has been aimed at obtaining accurate optical colors To obtain an accurate optical albedo from comparison and searching for correlations of color with various orbital of optical and infrared observations, an accurate optical ab- parameters (see, e.g., Tegler et al., 2003). In addition to opti- solute magnitude or flux value is required. Because of the cal colors, we have also obtained some information on light increasing importance of accurate values of optical parame- curves of individual objects (Romanishin and Tegler, 1999; ters, we present here such information for the objects for Romanishin et al., 2001). which we have published optical photometry. We also com- For all objects in our sample, we have measured accurate pare our derived HV values with those available from several V band magnitudes and V–R colors from at least one . large databases of properties of minor Solar System bodies. These data can, of course, be used to derive an estimate of the absolute visual magnitudes (HV) of the objects. The HV 2. Absolute magnitudes

* Corresponding author. Fax: +1 405 325 7557. E-mail address: [email protected] (W. Romanishin). We use the H , G formalism (Bowell et al., 1989) to derive 1 Observers at the Keck I, Bok, and Vatican Advanced Technology tele- the absolute visual magnitude (HV), which is the magnitude scopes. that would be observed in the V passband by an observer at

0019-1035/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2005.06.016 524 W. Romanishin, S.C. Tegler / Icarus 179 (2005) 523–526 a distance of 1 AU from an object that is 1 AU from the , Table 1 at a of 0◦. HV and HR magnitudes We derive HV from the observed V magnitude, distances, Number Name Prov. Des. H H and phase angle: V R

2003 CO1 9.29 8.80 HV = V − 5log(r)   2001 XZ255 11.24 10.49 + 2.5log (1 − G)Φ1(α) + GΦ2(α) , (1) 2001 SQ73 9.24 8.78 2001 QX322 6.70 6.10 where V is the measured magnitude of the object in V band, 2001 KG77 8.62 8.18 r is the heliocentric distance (in AU) at the time of obser- 2001 KC77 7.23 6.67 vation, is the geocentric distance (in AU) at the time of 2001 FM194 7.91 7.47 observation, α is the phase angle (Sun–target–observer an- 2000 KK4 6.46 5.82 gle) at time of observation and 2000 FZ53 11.72 11.16 2000 FS53 7.88 7.17     1 Bi 2000 CR105 6.60 6.08 Φi(α) = exp −Ai tan α , (2) 2000 CQ105 6.29 5.85 2 2000 CF105 7.59 6.89 1999 TR11 8.63 7.88 where i = 1, 2, A1 = 3.33, B1 = 0.63, A2 = 1.87, B2 = 1999 HV11 7.47 6.88 1.22. 1999 HS11 6.88 6.16 We assume a G value of 0.15, which is the value most 1999 CF119 7.42 6.81 often adopted for this class of objects. Using observations 1998 WX24 6.79 6.09 = 1998 WV24 7.43 6.93 over a range of α, Buie and Bus (1992) derive G 0.16 for 1998 KS65 7.63 6.99 (5145) Pholus. Unfortunately, almost all of our observations 1998 FS144 7.17 6.60 of individual objects are made at a single epoch, so we can- 1997 SZ10 8.75 8.10 not constrain G values. 1997 QH4 7.44 6.77 Observed V magnitudes and observation dates were 1997 CV29 7.71 7.06 1997 CT29 7.19 6.44 taken from our published papers (Tegler and Romanishin, 1996 TS66 6.50 5.74 1998, 2000, 2003; Romanishin and Tegler, 1999; Tegler 1996 TQ66 7.69 6.99 et al., 2003). Distances (r and ) were derived from the 1996 TK66 6.75 6.12 JPL Horizons online database (Giorgini et al., 1996; http: 1996 RR20 7.20 6.49 //ssd.jpl.nasa.gov/horizons_doc.html). For objects with H 1996 RQ20 7.00 6.56 V 1995 HM5 8.29 7.88 listed in Romanishin and Tegler (1999),newHV values were 1994 TA 12.05 11.37 derived for consistency and also to use distances from 1993 RO 8.92 8.41 with any improvements made since that paper. The new HV 1993 FW 7.09 6.46 values for these objects agree to within 0.01 mag with our 95626 2002 GZ32 7.24 6.82 previously published values. 88269 2001 KF77 10.52 9.79 87269 2000 OO67 9.82 9.10 Our derived HV values are listed in Table 1.Forthe 86047 1999 OY3 6.46 6.09 majority of the objects, we have observations taken only 85633 1998 KR65 7.10 6.43 during one night, or perhaps a few nights during one ob- 83982 2002 GO9 9.17 8.41 serving run. The information in this table is available online 82158 2001 FP185 6.38 5.80 at http://observatory.ou.edu/kbos.html. Our color survey is 82155 2001 FZ173 6.23 5.68 82075 2000 YW134 4.74 4.19 continuing, and we will update the online information as we 79360 1997 CS29 5.52 4.91 derive final magnitudes and colors for newly observed ob- 73480 2002 PN34 8.66 8.14 jects. For convenience we also list HR values in Table 1, 65489 2003 FX128 6.60 6.04 63252 2001 BL41 11.46 10.95 derived from the HV values and published V–R colors. 60621 2000 FE8 6.83 6.35 60608 2000 EE173 8.49 8.00 60458 2000 CM114 7.36 6.86 3. Comparison with other sources 55636 2002 TX300 3.47 3.11 2000 QC243 7.69 7.19 Another large photometric survey of minor outer Solar 52975 Cyllarus 1998 TF35 8.99 8.24 System bodies is available from a collaboration of a number 1998 SG35 11.23 10.75 52747 1998 HM151 8.02 7.40 of observers using European Southern Observatory (ESO) 50000 Quaoar 2002 LM60 2.74 2.10 telescopes. They present HV values (Doressoundiram et al., 49036 Pelion 1998 QM107 10.54 10.02 2002) or HR values (Peixinho et al., 2004), which we con- 47171 1999 TC36 5.33 4.64 44594 1999 OX3 7.85 7.16 verted to HV values by adding their V–R color for each ob- ject. Ignoring a few objects in these lists with large (greater 42355 2002 CR46 7.65 7.13 (continued on next page) than 0.1) errors in HV, we find 30 objects in common with Absolute magnitudes of KBOs 525

Table 1 (continued) differences between the HV values listed in Table 1 and

Number Name Prov. Des. HV HR those obtained from the other databases. The distribution of 2000 EB173 5.03 4.43 magnitude differences are shown in Fig. 1. The average dif- 38084 1999 HB12 7.04 6.47 ference between JPL and ours (HV (JPL) − HV (ours)) is 33001 1997 CU29 6.68 6.02 −0.34 (s = 0.33). The average difference between MPC and 32929 1995 QY9 8.06 7.59 ours (HV (MPC) − HV (ours)) is −0.29 (s = 0.34). Thus, 2001 PT13 9.32 8.85 these HV magnitudes are significantly brighter than ours. 1999 UG5 10.49 9.82 29981 1999 TD10 9.06 8.59 To find the origin of these systematic offsets in HV val- 26375 1999 DE9 5.20 4.62 ues, one would have to carefully review the input magni- 26308 1998 SM165 6.13 5.38 tudes into the databases for each object. The vast majority 24978 1998 HJ151 7.67 6.96 of observed magnitudes for minor bodies in the outer Solar 24835 1995 SM55 4.54 4.15 System in these databases presumably come from observa- 20108 1995 QZ9 8.58 8.06 2000 WR106 3.92 3.36 tions primarily taken for either discovery or of 1998 WH24 4.94 4.32 these objects. Such observations are often taken using non- 19308 1996 TO66 4.76 4.38 standard filters (see, e.g., Millis et al., 2002), photometric 19299 1996 SZ4 8.44 7.92 calibrations are not always made, and less than photomet- 19255 1994 VK8 7.56 6.89 ric observing conditions can be utilized. We stress that the 16684 1994 JQ1 7.14 6.51 15875 1996 TP66 7.39 6.71 magnitudes in the MPC and Horizons databases are from a 15874 1996 TL66 5.39 5.04 number of different sources, and that some of these undoubt- 15820 1994 TB 8.07 7.39 edly have better magnitude values than others. 15810 1994 JR1 7.35 6.99 For minor Solar System bodies, the combination of opti- 15789 1993 SC 7.26 6.56 cal flux, related to the amount of reflected from the 15788 1993 SB 8.15 7.68 15760 1992 QB1 7.61 6.83 body, and thermal infrared flux, from the thermal emission 10370 Hylonome 1995 DW2 9.53 9.10 of the body, can, of course, yield the and optical albedo 1997 CU26 6.76 6.28 of the body (see, e.g., Harris and Lagerros, 2002, and ref- 1995 GO 9.18 8.71 erences therein). How would a mis-estimate of the optical 1993 HA2 9.52 8.75 magnitude and hence optical flux affect the results of this 1992 AD 7.63 6.85 technique? Detailed application of this technique requires a thermal model of the body, so a precise answer of this ques- our sample presented in Table 1. The mean difference be- tion is not simple. However, because the albedo (A) of these tween the ESO collaboration HV values and ours for these outer Solar System bodies is assumed to be low, it is easy to 30 objects is −0.01 (s = 0.13) mag. A histogram of the see the approximate effects. As the infrared emission is re- differences is presented in Fig. 1. Twenty two out of 30 dif- lated to the amount of sunlight absorbed by the body, which ferences are 0.1 mag or less in absolute value, while the is goes as 1 − A, even a significant fractional change in A remaining differences trail out a few tenths of a magnitude to (if A is small compared to 1) will result in only small frac- both positive and negative sense. This histogram shows that tional change in 1 − A. Thus, a given fractional overestimate there is reasonable overall agreement between our HV val- in the optical flux will primarily result in the same fractional ues and those of the ESO collaboration. Those objects with overestimate in the derived optical albedo. differences greater than 0.1 mag may be objects that have lightcurves with amplitude of greater than 0.1 mag that were observed at different parts of their lightcurve by different ob- 4. Conclusions servers. The combination of our survey and the ESO survey con- We have presented accurate HV values for a sizable sam- tains photometry for about 160 unique objects, approxi- ple of outer Solar System objects. Comparison of our values mately 30% of the currently known population with mul- with those of the European collaboration photometric sur- tiple opposition orbits. Several online databases contain or- vey shows good agreement. Comparison of these values with bital and physical properties of much larger samples of ob- those of several databases of general object properties shows jects: the JPL Horizons system, referenced previously and significant systematic differences, with our values predomi- the MPC ( Center) databases (http://cfa-www. nately fainter. harvard.edu/iau/lists/MPLists.html). These sites list absolute Obviously, the ideal situation to compare optical and in- magnitude values derived mostly using photometry derived frared photometry would be to have truly simultaneous ob- from discovery or astrometry observations. servations at all wavelengths. Such coordination is usually Because these sites give at least some information for not possible. Our observed magnitudes are carefully cali- every known object, they are important, particularly for sta- brated in standard filters. If it is not possible to obtain new tistical studies. To check the accuracy of the absolute mag- calibrated optical photometry, values from our survey, or de- nitudes listed on these sites, we computed the magnitude rived from magnitudes given by the European survey, should 526 W. Romanishin, S.C. Tegler / Icarus 179 (2005) 523–526

Fig. 1. (a) Histogram of differences between HV magnitudes from the ESO collaboration and those reported here for 30 objects. (b) Histogram of differences between HV magnitudes from the JPL Horizons site and those reported here for 90 objects. (c) Histogram of differences between HV magnitudes from the MPC site and those reported here for 90 objects. be used if available for the objects of interest. If absolute Harris, A.W., Lagerros, J.S.V., 2002. in the thermal infrared. In: magnitudes from other databases must be used, the system- Bottke, W.F., Cellino, A., Paolicchi, P., Binzel, R.P. (Eds.), Asteroids atic offsets reported here can be used to obtain more accurate III. Univ. of Arizona Press, Tucson, pp. 205–218. Giorgini, J.D., Yeomans, D.K., Chamberlin, A.B., Chodas, P.W., Jacobson, HV values. However, due to the scatter and heterogeneous R.A., Keesey, M.S., Lieske, J.H., Ostro, S.J., Standish, E.M., Wimberly, sources of the magnitudes in these databases, the use of these R.N., 1996. JPL’s online Solar System data service. Bull. Am. Astron. magnitudes for purposes such as measuring optical Soc. 28, 1158. is strongly discouraged. Millis, R.L., Buie, M.W., Wasserman, L.H., Elliot, J.L., Kern, S.D., Wag- ner, R.M., 2002. The deep survey: A search for Kuiper belt objects and Centaurs. I. Description of methods and initial results. As- Acknowledgments tron. J. 123, 2083–2109. Peixinho, N., Boehnhardt, H., Belskaya, I., Doressoundiram, A., Barucci, We thank the NASA Planetary Astronomy program for M.A., Delsanti, A., 2004. ESO large program on Centaurs and TNOs: financial support of this research and the NASA-Keck, Stew- Visible colors—Final results. Icarus 170, 153–166. ard Observatory, and Vatican Observatory Telescope Alloca- Romanishin, W., Tegler, S.C., 1999. Rotation rates of Kuiper belt objects tion Committees for consistent allocation of telescope time. from their lightcurves. 398, 129–132. Romanishin, W., Tegler, S.C., Rettig, T.W., Consolmagno, G., Botthof, B., 2001. 1998 SM 165: A large Kuiper belt object with an irregular shape. References Proc. Nat. Acad. Sci. 98, 11863–11866. Tegler, S.C., Romanishin, W., 1998. Two distinct populations of Kuiper belt Bowell, E., Hapke, E.B., Domingue, D., Lumme, K., Peltoniemi, J., Harris, objects. Nature 392, 49–51. A.W., 1989. Application of photometric models to asteroids. In: Binzel, Tegler, S.C., Romanishin, W., 2000. Extremely red Kuiper belt objects in R.P., Gehrels, R., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona near-circular orbits beyond 40 AU. Nature 407, 979–981. Press, Tucson, pp. 524–556. Tegler, S.C., Romanishin, W., 2003. Resolution of the Kuiper belt object Buie, M., Bus, S.J., 1992. Physical observations of 5145 Pholus. Icarus 100, color controversy: Two distinct color populations 2003. Icarus 161, 288–294. 181–191. Doressoundiram, A., Peixinho, N., De Bergh, C., Fornasier, S., Thebault, Tegler, S.C., Romanishin, W., Consolmagno, G.J., 2003. Color patterns in P., Barucci, M.A., Veillet, C., 2002. The color distribution in the the Kuiper belt: A possible primordial origin. Astrophys. J. 599, L49– Edgeworth–Kuiper belt. Astron. J. 124, 2279–2296. L52.